Prevention of splattering during vaporization processing



United States Patent 3,356,487 PREVENTION OF SPLATTERING DURING VAPORIZATION PROCESSING Charles dA. Hunt, On'uda, and Jack D. Merrill, Richmond, Califi, assignors, by mesne assignments, to Air Reduction Company, Incorporated, a corporation of New York No Drawing. Filed Dec. 6, 1966, Ser. No. 599,415 7 Claims. (Cl. 7510) This application is a continuation-in-part of Serial No. 288,822, filed June 18, 1963, now Patent No. 3,330,647.

The present invention relates generally to vacuum vaporization processes and more particularly to the prevention of splattering during electron beam vacuum vaporization of metals, and is particularly useful in connection with very rapid rates of vaporization.

A serious problem in the vacuum vaporization of metals is the explosive vaporization or splattering which is often experienced. This problem is particularly acute in electron beam vaporization processes where the surface of a molten pool of metal is heated by a high energy electron beam. In electron beam vaporization processes all of the heat is added to the metal being vaporized at the surface thereof by impingement of the electron beam upon the surface of the molten meal. The impingement of the electron beam on the surface of the molten metal causes thermal stirring of the molten pool which continuously brings fresh metal to the surface of the pool where vaporization takes place.

It has been discovered through considerable experience that when a metal is vaporized by an electron beam vaporization process there is a tendency for the metal to throw off globules of liquid metal during the vaporization process. This phenomenon is generally attributed to the fact that electron beam heating which is concentrated at the surface of the molten pool, causes localized superheating of regions of the molten'pool at and near the surface. These localized superheated regions of the melt are constantly moving due to convection currents produced by thermal gradients in the pool. If a superheat region reaches an irregularity at the surface of the pool, it may be explosively vaporized depending upon the amount of excess energy present and the particular characteristics of the melt. Irregularities at the surface may exist at particles of impurities floating in the pool and at a meniscus at the edge of the pool. The explosive vaporization of the superheated region is accompanied by splattering of the globules of molten metal from the pool. The amount of splattering relative to the amount vaporized generally increases with increasing vaporization rates, and in some instances the amount of splattering which has occurred in the past has prevented the utilization of electron beam vacuum vaporization of many metals. The problem of splattering, which is present even at low vaporization rates and which is magnified at higher vaporization rates, is substantially avoided by the present invention.

Although the present invention is applicable to improve all vacuum vaporization processes, it is primarily important in the vacuum vaporization of metals having relatively low or intermediate melting temperatures. As used herein, the term metal is intended to include alloys as well as pure metals since the same problems of splattering occur.

It is a principal object of this invention to provide an improved vacuum vaporization method in which splattering is reduced. Another object is to provide an electron beam vacuum vaporization process by which metals may be vaporized at high vaporization rates and with reduced splattering. A further object is to provide a continuous electron beam vacuum vaporization process by 3,356,487 Patented .Dec. 5, 1967 which a metal to be vaporized may be continuously introduced into a crucible and vaporized using electron beam heating at high vaporization rates with reduced amounts of splattering.

Additonal objects and advantages of the invention will become apparent from the following detailed description.

Very generally, the present invention is directed to an improved electron beam vaporization process in which a metal is vaporized from a molten pool by utilizing electron beam heating. In order to prevent splattering, a minor amount of an additive metal forms a single phase molten solution with the metal to be vaporized, thereby lowering the surface tension of the molten pool and allowing the molten metal to be vaporized at high vaporization rates without splattering.

The additive metal should have certain characteristics in order to prevent splattering. First, the additive metal should have a lesser density than the metal to be vaporized in order that it will rise to the surface of the molten metal and form a layer or stratum adjacent the surface of the molten pool which contains a high concentration of additive metal. The additive metal should also have a substantially lower vapor pressure than the metal to be vaporized at the operating conditions of the vacuum vaporization process in order that significant amounts of the additive metal will not be vaporized, unless, of course, it is desired that a portion of the additive metal be vaporized to provide an alloy product.

It has been discovered that the addition of a minor amount of an additive metal to a molten pool of metal being vaporized by electron beam heating substantially reduces the splattering normally associated with this form of vaporization. The exact nature of the phenomenon which occurs Within the molten pool is not altogether understood, but it is believed that what occurs is the lowering of the surface tension of the surface of the molten pool which allows the metal molecules being vaporized to more easily escape from the surface of the molten pool at lower energy levels. This in turn allows for the majority of the metal molecules to be vaporized before they become significantly superheated, and accordingly, splattering is greatly reduced. The exact manner in which the surface tension is reduced is also not fully understood. Experience has shown that in substantially all instances a single phase solution of a molten metal and an additive metal has a lower surface tension than the pure molten metal. It is postulated that this occurs because the molecules of the additive metal, When mixed with the metal to be vaporized, become interspersed between the metal molecules. It is believed that interpositioning of the molecules of additive metal between the molecules of the metal to be vaporized destroys, or substantially reduces, the surface bonding forces between the molecules of the metal being vaporized. This lowers the surface tension and allows the escape of metal molemiles from the molten pool at lower energy levels and without excessive superheating.

The problem of splattering in Vacuum vaporization processes was encountered when metals were vaporized using resistance wire heating. In such processes clips of metal to be vaporized are hung on a resistance wire heater which is heated by passage of a current therethrough to vaporize the metal. It was found that splattering in the resistance wire heater process occurred for two principal reasons, the metal being vaporized did not wet the resistance wire snfliciently or wet the resistance wire too easily. In the case where the 'metal did not wet the wire sufficiently, the metal hung in drops on the wire. Since the metal was being heated from the bottom, globules of molten metal were thrown off during vaporization due to the thickness of the metal layer on the Wire. This problem was solved by adding an alloying agent to the metal which preferentially Wet the surface of the resistance wire and caused the metal to spread out on the wire in a thin coating from which vaporization occurred with less splattering. In the case where the metal wets the wire too rapidly, exactly the opposite phenomenon occurred. The metal being vaporized wet the wire so rapidly that it spread out on the Wire too fast and caused splattering. Here the alloying agent was added to slow down the wetting of the wire. In each of these cases the purpose of the addition of the alloying agent was to increase or decrease the wetting ability of the metal to be vaporized in respect to the heater wire.

The present invention is not concerned with the wetting characteristics of the metal being vaporized. The addition of the additive metal of particular characteristics allows the metal to be vaporized at reduced energy levels and without superheating.

In accordance with the method of the invention, a metal to be vaporized is introduced into a suitable crucible may be a graphite or other refractory crucible, or or more electron beams, generally in accordance with vacuum vaporization technology. For example, the crucible may be a graphite or other refractory crucible, or may be a water cooled crucible. The crucible is disposed within a vacuum chamber which is evacuated to a pressure of 1.0 milliTorr or lower, preferably below 0.1 milliTorr. The crucible may be of any convenient size and shape depending upon the amount and rate of vaporization desired. The crucible may, for example be from 2 inches to 6 inches in diameter and may have a depth of from /2 inch to 2 inches for usual processes. In most commercial processes fresh metal is continuously fed into the crucible for continuous vaporization. The material may be added in molten form or in solid form, for example, in the form of wire.

The present invention is principally directed to the vaporization of nonrefractory metals, since problems of splattering are not generally encountered in the vaporization of refractory metals. Generally, the nonrefractory metals to which the application pertains have melting points below 3000 F. Specific examples of nonrefractory metals which can be vaporized in accordance with the invention with substantially reduced amounts of splattering include copper, iron, cobalt, nickel, chromium, gold, and silver. As previously indicated alloys of these and other nonrefractory metals in which the nonrefractory metal is the base metal are contemplated. Thus, the invention contemplates the vaporization of iron base stainless steel alloys with reduced splattering.

The additive metal is added to the molten metal to be vaporized in any convenient fashion. In this connection, the additive metal may be placed in the crucible before the metal to be vaporized is introduced into the crucible, or can be added, in the desired proportion, to the molten metal in the crucible before vaporization is begun. It has been found that reduction of splattering occurs with the addition of as little as 1 percent by weight of the additive metal, and the addition of about 5 percent to about percent by weight provides good results. The addition of the additive metals in amounts exceeding about 10 percent by weight do not appear to provide commensurate improvement in the process, and is generally not done. In some instances it should be understood that the desired evaporant may be an alloy of the metal and the additive metal. In such instances operating conditions are adjusted in accordance with known technology in order to vaporize a portion of the additive metal. In such instances it is necessary to replenish the additive metal in the molten pool in order to maintain the desired concentration of additive metal in the molten pool.

The additive metal should form a single phase solution with the metal to be vaporized and should have a lesser density than the metal to be vaporized in order that the additive metal will be concentrated in a layer or stratum adjacent the top surface of the molten pool. It has been found that if the additive metal has a greater density than the metal to be vaporized, and therefore, sinks to the bottom of the molten pool, the problem of splattering may not be alleviated. However, when the additive metal is concentrated in a layer adjacent the surface of the molten pool, splattering is substantially reduced.

The concentration of the additive metal will be the greatest in the region where the electron beam impinges on the surface of the molten pool. This region is generally in the center of the pool. This is the area from which a major amount of the molten metal is vaporized and consequently the pool is being depleted of the metal in this region. Consequently, the disruptive nature of the presence of the molecules of additive metal is greatest, due to an increased concentration, in this region. This causes the surface tension to be lower in this region of the surface, which means that a molecule of metal can escape from the surface at a lower energy level in this region than in any other region of the surface. When fresh molten metal enters this stratum, its surface bonding forces are immediately reduced and the metal is vaporized before it can become significantly superheated.

The additive metal should have a vapor pressure that is sufiiciently lower than the metal to be vaporized so that the desired metal can be conveniently vaporized from the molten pool at rates without significant vaporization of the additive metal which would act as in impurity in the condensed metal product. Obviously, the presence of minute amounts of additive metal in most products can be tolerated. As indicated, in some instances the additive metal might be a desired alloying agent in the product. In such instances the concentration of additive is raised until the desired amount of additive metal is vaporized along with the base metal. In such instances the additive metal is replenished in the molten pool as needed to maintain the desired concentration.

It can be seen that the additive metal should have particular characteristics in order to reduce splatter in the process. Zirconium has these particular properties and has been found to be particularly suitable for use as an additive metal. Other metals which form a single phase solution, have a lesser density, and have a higher vapor pressure than the metal to be vaporized could also be utilized. The choice of metals, other than zirconium, which are useful in the invention can readily be made by those of ordinary skill in the art.

In an example of the invention copper was vaporized from a graphite crucible 6 inches in diameter and 2 inches deep using an electron gun having a power output of 5S kilowatts. The copper was heated to a temperature of about 3000" F. and a vapor flow rate of about 20 pounds per hour was obtained. The copper vapors were condensed on a stainless steel substrate positioned 14 inches above the crucible.

The copper was first vaporized without the addition of an additive metal. A substantial amount of splattering occurred and the coated substrate was considered to be of low quality because of this splatter.

About 7 percent by weight of zirconium was then added to the molten pool of copper and the vaporization was continued at a temperature of 3200 F. The splattering of molten copper from the pool was substantially completely eliminated and the coating was judged to be much superior to the coating obtained without the addition of zirconium.

Similar results were obtained in the vacuum vaporization of iron, stainless steel, cobalt, nickel and other nonrefractory metals. The addition of the additive metal reduced the amount. of splattering in the electron beam vacuum vaporization process and provided an improvement over prior processes.

It will be seen that an improved electron beam vaporization process has been disclosed whereby reduced splattering occurs. Although certain features of the invention are described with particularity, alternative embodiments within the skill of the art are contemplated.

Various of the features of the invention are set forth in the following claims.

What is claimed is:

1. An improved electron beam vacuum vaporization process for vaporizing a metal from the upper surface of a molten pool comprising, heating a metal with an electron beam to form a molten pool, adding an additive metal to the molten pool to reduce surface tension at the upper surface of the molten pool, the additive metal having a lesser density and substantially lower vapor pressure than the metal, and heating the surface of the molten pool with the electron beam to a temperature suflicient to vaporize the metal, whereby the metal may be rapidly vaporized without splattering.

2. A method in accordance with claim 1 wherein the metal to be vaporized has a melting temperature below 3000 F.

3. A method in accordance with claim 1 wherein the additive metal is present in the molten pool in an amount of between about 1 percent and about percent by weight.

4. A method in accordance with claim 1 wherein amounts of the metal to be vaporized are fed into the molten pool during the process to replenish vaporized metal.

5. A method in accordance with claim 1 wherein the metal to be vaporized is selected from the group consist ing of copper, iron, cobalt, nickel, chromium, gold, silver and mixtures thereof.

6. A method in accordance with claim 1 wherein the additive metal is zirconium.

7. A method in accordance with claim 1 wherein the metal to be vaporized is copper and the additive metal is zirconium.

References Cited UNITED STATES PATENTS 2,450,856 1-0/ 1948 Colbert et al. 117107 X 2,589,175 3/1952 Weinrich 117-107 X 2,665,223 1/ 1954 Clough et al 118-49 X 2,665,225 1/1954 Godley 118-49 X 2,665,229 1/1954 Schuler et all. 11849 X 2,731,365 1/1956 Weinrich 117107 2,962,538 11/1960 Alexander 117-107 X 2,963,530 12/1960 Honks et al. 1331 3,058,842 10/1962 Kahan et a1 11849 X 3,068,337 12/1962 Kuebrich et al. 117-107 X 3,084,037 4/1963 Smith 10 3,152,246 10/1964 Van Deuren et a1. 117107 X 3,181,209 5/1965 Smith 11849 X 3,205,087 9/ 1965 Allen 117-107 X DAVID L. RECK, Primary Examiner. H. W. TARRING, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,356,487 December S, 1967 Charles d'A. Hunt et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line 20, for "may be a graphite or other refractory crucible or" read and is heated to form a molten pool by means of one Signed and sealed this 31st day of December 1968.

(SEAL) Attest:

Edward M. Fletcher, Jr. EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. AN IMPROVED ELECTRON BEAM VACUUM VAPORIZATION PROCESS FOR VAPORIZING A METAL FROM THE UPPER SURFACE OF MOLTEN POOL COMPRISING, HEATING A METAL WITH AN ELECTRON BEAM TO FORM A MOLTEN POOL,A DDING AN ADDITIVE METAL TO THE MOLTEN POOL TO REDUCE SURFACE TENSION AT THE UPPER SURFACE OF THE MOLTEN POOL, THE ADDITIVE METAL HAVING A LESSER DENSITY AND SUBSTANTIALLY LOWER VAPOR PRESSURE THAN THE METAL, AND HEATING THE SURFACE OF THE MOLTEN POOL WITH THE ELECTRON BEAM TO A TEMPERATURE SUFFICIENT TO VAPORIZE THE METAL, WHEREBY THE METAL MAY BE RAPIDLY VAPORIZED WITHOUT SPATTERING. 